The invention relates to a method of detecting a sequence of information symbols from a first signal subjected to inter-symbol interference, wherein each symbol can adopt one of a number of different values, said method being performed as one or more signal processing paths, and wherein the following steps are performed repetitively:
setting, in each of said one or more signal processing paths, a symbol in the sequence to a value based on an intermediate signal derived from said first signal and a feedback signal generated on the basis of one or more previously set symbols,
dividing a signal processing path, in which said intermediate signal for a given symbol exceeds a given threshold, into two separate signal processing paths, setting said given symbol to different values in each of said two separate signal processing paths, and
selecting, after setting a number of symbols, the sequence of information symbols from one of said one or more signal processing paths as the detected sequence of information symbols.
The invention also relates to a mobile station configured to perform the method.
When transmitting a speech signal or a data signal from a first communications device to a second communications device in a radio communications system, such as a mobile telephone communications system, the signal typically propagates along several different paths due to obstacles and reflections. Therefore, the signal arrives at the second communications device as several signal components with different delays. This phenomenon, which is normally called multi-path propagation, which may cause so-called inter-symbol interference or ISI where successive symbols are partly superimposed. This effect is undesirable as it complicates demodulation of the received signal, e.g. in the mobile station. Therefore, different kinds of equalization devices have been used in order to eliminate or reduce inter-symbol interference, e.g. a Maximum-Likelihood Sequence Estimator (MLSE) or Decision-Feedback Equalizer (DFE).
From the prior art, it is also known to perform a detection of a sequence of information symbols from a signal subjected to inter-symbol interference using a so-called parallel decision-feed back equalizer or PDFE. In such an equalizer, which is an example of a detector of the above-mentioned type, information about previously detected symbols is used when detecting new symbols. It is noted that each of the symbols being detected from the signal can adopt one of a number of different values, e.g. the values representing “0” and “1” when the first signal includes information in binary form. Therefore, the task of the detector is to determine which one of the possible values should be selected when generating an output signal or a sequence of information symbols detected.
The detection is performed in one ore more so-called signal processing paths wherein a number of signal processing steps are performed on basis of a given input signal. Due to noise, the decision which value a symbol should be assigned may be difficult to make, i.e. the decision is uncertain, as the distance between the value of the signal to be detected and the possible symbol values exceeds a given threshold. According the prior art, this problem is solved by splitting the signal processing path into two when an uncertain a decision has been taken.
Normally, in environments where the receiver is subjected to low-level noise, the equalizer may operate by use of a single signal processing path only. As the level of the noise increases, a situation wherein an uncertain decision is taken, i.e. when the distance between the value of the signal to be detected and the nearest symbol value exceeds a given pre-defined threshold, may occur. In order to solve this problem, the signal processing path is split into two, when an uncertain decision has been taken. One path uses the best decision as the symbol, i.e. the nearest symbol value is selected as the symbol value used, and the other signal path uses the second best decision as the symbol.
The detection is now performed as two signal processing paths performed simultaneously. If, an uncertain signal is received in either the first or the second path, then this path is split into two paths, and hereafter three signal processing paths are performed simultaneously. In this way, the signal processing path branches, when needed. When a given number of symbols has been detected, the sequence of symbols from one of the paths is selected to be the detected sequence of symbols, e.g. the sequence having the smallest squared error sum is selected. Hereafter, the scenario may continue or start over again.
According to the prior art, a fixed value is assigned to the above-mentioned threshold, which is optimized to the noise level in the environment in which the apparatus is to be operated. The noise may be of varying natures, e.g. remaining ISI due to a non-ideal ISI-cancelling, noise added by the sender filter, noise added by the receiver filter, and/or additional channel noise of various kinds. This solution may be adequate for many purposes, but when used in a computation limited and/or storage capacity limited apparatus—such as a mobile station—it has the shortcoming of not providing an optimal solution when the noise level is increased. The reason shall be found in the fact that, as a result of the increased noise level, the detector will start new signal processing paths more frequently as more uncertain decision are taken due to the increased noise level. In the situation when all the available signal processing paths are in use, no signal processing paths are available if/when an uncertain decision occurs. When this situation occurs the performance will degrade as uncertain decision are taken—in fact it is not possible to start a new signal processing path even when the degree of uncertainty of the most recently detected symbol is high.